In some applications, it is necessary to measure, by interferometry, absolute distance or displacement of a moving target from a fixed reference point. Current laser tracker systems use a gimbal mounted mirror to steer the interferometer beam to follow the moving target (see FIG. 1). In practice, the moving target is provided with a retro-reflector which reflects the laser beam back towards the tracking system Movement of the target along the interferometer axis is sensed by the interferometer--the interferometer axis being the measurement beam. Any component of the target's motion that is perpendicular to the interferometer axis causes the position of the returned beam to change. This change in position is sensed using a conventional position sensitive detector (PSD) to provide error signals which indicate the amount of movement which has occurred in two orthogonal axes. The system not only obtains a measure of the movement of the target along the interferometer axis but also redirects the interferometer beam to follow the target as the target moves. In this way tracking of the target can be achieved on a continuous basis.
The laser tracker relies on there being a fixed point somewhere within the system that can be used as a datum for distance or displacement measurement. If this datum is ill-defined or moving, then measurement errors will result.
The problem with the above-mentioned method of beam steering is that the measurement datum is defined by a number of mechanically un-related systems. These are the two axes of rotation, the plane of the reflection mirror and the point of reflection of the laser beam from the mirror.
Defining a perfect datum from these parts requires:
the two rotation axes to be aligned such that they intersect at a point; PA1 the point of intersection of the axes to lie on the plane of the mirror; PA1 the steered laser beam to be reflected off the mirror at the point of intersection of the rotation axes and the plane of the mirror.
If any of these conditions is not met then the resulting datum is not a point but is contained within a volume. This leads to distance or displacement measurement error.
To reduce distance measurement errors to below the 1 .mu.m level requires all these alignments to be made better than 1 .mu.m. In practice this is difficult if not impossible to achieve and maintain.